As is known in the art, battery longevity is a primary concern in implanted medical devices (also referred to simply as “implants”) due to the significant cost and risk of additional surgery required to replace a battery. Battery longevity is, in turn, highly sensitive to the accuracy of the final charging voltage on the battery. Lithium-ion (Li-ion) batteries are a popular choice for implants due to their ability to provide relatively high performance in both energy and power densities, of 158 Wh/Kg and 1300 WlKg, respectively.
As is also known, lithium-ion (Li-ion) batteries have a target voltage of 4.2 volts (V). It has been shown that undercharging a lithium-ion (Li-ion) battery by 1.2% of the 4.2 V target value results in a 9% reduction in battery capacity. Conversely, if a Li-ion battery is overcharged, dangerous thermal runaway can occur. During discharge, deeply discharging the Li-ion battery below 3 V can permanently reduce the cell's capacity.
As is also known, conventional Li-ion charger designs often suffer from two significant problems. First, unnecessarily complex control circuitry is often employed to manage battery charging at the expense of circuit area and power consumption. Second, many charger circuits require a sense resistor to detect end-of-charge. This latter point is particularly problematic for battery longevity due to the challenges of precision on-chip resistor fabrication, as undercharging the battery can drastically reduce its capacity
Referring now to FIG. 1, a plot of current and voltage vs. time illustrates a charging profile of a Li-ion battery. Curve 8 corresponds to current and curve 10 corresponds to voltage. As can be seen from FIG. 1, the charging profile can be divided into four distinct regions: a trickle-charge region 12, a constant current region 14, a constant voltage region 16, and an end-of-charge region 18. Trickle charging is required only if the battery is deeply discharged (voltage is less than 3 V). During trickle-charge, the battery is charged with a small amount of current, typically no more than 0.1 times the rated capacity of the battery, or (0.1C) where C represents the battery capacity expressed in terms of amperehours (Ah). Charging currents greater than O.1C may be hazardous as Li-ion batteries typically have a relatively high internal impedance at such low voltages. Above 3.0 V, the battery may be charged at higher currents; this is the constant current region 14. As the battery voltage approaches 4.2 V, the charging profile enters the constant voltage region 16. In this region, the charging current should be progressively decreased as the battery voltage approaches 4.2 V.
The constant voltage region 16 is required in order to compensate for internal battery voltage drop; as the charging current decreases, the battery output voltage also decreases due to lower voltage drop across its internal impedance. Charging current should be decreased until a certain threshold is met, which is usually about 2% of the rated battery capacity. Once this charging current is reached, the charger enters the end-of-charge region 18 in which no current is provided.